The link layer in this section will comprise all protocols which
support the ATM layer.
The wireless data link layer
will be adaptive to provide an appropriate degree of data rate versus
reliability in order to properly support the various types of ATM
traffic. For example, we may want to drop voice packets, which are very
time sensitive, but retry data packets. The edge interface unit has some
knowledge of the requirements of each traffic stream. For example this
knowledge can be obtained from VC identification. The efficiency of
the data link layer will be improved by using this knowledge.

In this architecture ATM will be carried end-to-end. However, at the
edge between the wired (high-speed) network and wireless links,
multiple ATM cells will be combined in an HDLC-like frame. For some types
of traffic, error correction may be achieved using
retransmission. Here, delay is increased for this class of traffic to
prevent cell losses. It is well known that even a few cell losses can
have a significant impact on the performance of TCP/IP, while TCP/IP
can cope with variable delays [1]. The HDLC-like protocol can
change in response to traffic
requirements. ATM end-to-end provides the following benefits:

Moderate cut-through, eg. an IP segment may contain 8192
bytes or about 170 cells, while one ATM HDLC-like frame will contain on the
order of 3-20 cells

ATM is a standard protocol.

ATM can incorporate standardized QoS parameters based on
such things as VC number.

The link layer must also maintain cell order; this will be critical
during handoff of an RN from one ES to another. This is covered
further in Section 10. Figures 5 and 6
show the architecture
for the link/ATM level implementation. Below the device independent
ATM layer are the high speed packet radio (PR) specific layers.
The RN/ES interface bridges at
the cell level, not the AAL layer. No link level addressing is used since
the ATM level specifies the address.